Catalytically cracked gasoline forms a major part of the gasoline product pool in the United States. When the cracking feed contains sulfur, the products of the cracking process usually contain sulfur impurities which normally require removal, usually by hydrotreating, in order to comply with the relevant product specifications. These specifications are expected to become more stringent in the future, possibly permitting no more than about 300 ppmw sulfur (or even less) in motor gasolines and other fuels. Although product sulfur can be reduced by hydrodesulfurization of cracking feeds, this is expensive both in terms of capital construction and in operating costs since large amounts of hydrogen are consumed.
As an alternative to desulfurization of the cracking feed, the products which are required to meet low sulfur specifications can be hydrotreated, usually using a catalyst comprising a Group VIII or a Group VI element, such as cobalt or molybdenum, either on their own or in combination with one another, on a suitable substrate, such as alumina. In the hydrotreating process, the molecules containing the sulfur atoms are mildly hydrocracked to convert the sulfur to inorganic form, hydrogen sulfide, which can be removed from the liquid hydrocarbon product in a separator. Although this is an effective process that has been practiced on gasolines and heavier petroleum fractions for many years to produce satisfactory products, it does have disadvantages.
Cracked naphtha, as it comes from the catalytic cracker and without any further treatments, such as purifying operations, has a relatively high octane number as a result of the presence of olefinic components and as such, cracked gasoline is an excellent contributor to the gasoline octane pool. It contributes a large quantity of product at a high blending octane number. In some cases, this fraction may contribute as much as up to half the gasoline in the refinery pool.
Other highly unsaturated fractions boiling in the gasoline boiling range, which are produced in some refineries or petrochemical plants, include pyrolysis gasoline produced as a by-product in the cracking of petroleum fractions to produce light olefins, mainly ethylene and propylene. Pyrolysis gasoline has a very high octane number but is quite unstable in the absence of hydrotreating because, in addition to the desirable olefins boiling in the gasoline boiling range, it also contains a substantial proportion of diolefins, which tend to form gums after storage or standing.
Hydrotreating these sulfur-containing cracked naphtha fractions normally causes a reduction in the olefin content, and consequently a reduction in the octane number; as the degree of desulfurization increases, the octane number of the gasoline boiling range product decreases. Some of the hydrogen may also cause some hydrocracking as well as olefin saturation, depending on the conditions of the hydrotreating operation.
Various proposals have been made for removing sulfur while retaining the olefins which make a positive contribution to octane. Sulfur impurities tend to concentrate in the heavy fraction of the gasoline, as noted in U.S. Pat. No. 3,957,625 (Orkin) which proposes a method of removing the sulfur by hydrodesulfurization of the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution from the olefins which are found mainly in the lighter fraction. In one type of conventional, commercial operation, the heavy gasoline fraction is treated in this way. As an alternative, the selectivity for hydrodesulfurization relative to olefin saturation may be shifted by suitable catalyst selection, for example, by the use of a magnesium oxide support instead of the more conventional alumina. U.S. Pat. No. 4,049,542 (Gibson) discloses a process in which a copper catalyst is used to desulfurize an olefinic hydrocarbon feed such as catalytically cracked light naphtha.
In any case, regardless of the mechanism by which it happens, the decrease in octane which takes place as a consequence of sulfur removal by hydrotreating creates a tension between the growing need to produce gasoline fuels with higher octane number and the need to produce cleaner burning, less polluting, low sulfur fuels. This inherent tension is yet more marked in the current supply situation for low sulfur, sweet crudes.
Other processes for treating catalytically cracked gasolines have also been proposed in the past. For example, U.S. Pat. No. 3,759,821 (Brennan) discloses a process for upgrading catalytically cracked gasoline by fractionating it into a heavier and a lighter fraction and treating the heavier fraction over a ZSM-5 catalyst, after which the treated fraction is blended back into the lighter fraction. Another process in which the cracked gasoline is fractionated prior to treatment is described in U.S. Pat. No. 4,062,762 (Howard) which discloses a process for desulfurizing naphtha by fractionating the naphtha into three fractions each of which is desulfurized by a different procedure, after which the fractions are recombined.
U.S. Pat. No. 5,143,596 (Maxwell) and EP 420 326 B1 describe processes for upgrading sulfur-containing feedstocks in the gasoline range by reforming with a sulfur-tolerant catalyst which is selective towards aromatization. Catalysts of this kind include metal-containing crystalline silicates including zeolites such as gallium-containing ZSM-5. The process described in U.S. Pat. No. 5,143,596 hydrotreats the aromatic effluent from the reforming step. Conversion of naphthenes and olefins to aromatics is at least 50 percent under the severe conditions used, typically temperatures of at least 400.degree. C. (750.degree. F.) and usually higher, e.g. 500.degree. C. (about 930.degree. F.). Under similar conditions, conventional reforming is typically accompanied by significant and undesirable yield losses, typically as great as 25 percent and the same is true of the processes described in these publications: C.sub.5 + yields in the range of about 50 to 85 percent are reported in EP 420 326. This process therefore suffers the traditional drawback of reforming so that the problem of devising a process which is capable of reducing the sulfur level of cracked naphthas while minimizing yield losses as well as reducing hydrogen consumption has remained.
U.S. Pat. No. 5,346,609 describes a process for reducing the sulfur of cracked naphthas by first hydrotreating the naphtha to convert sulfur to inorganic form followed by treatment over a catalyst such as ZSM-5 to restore the octane lost during the hydrotreating step, mainly by shape-selective cracking of low octane paraffins. This process, which has been successfully operated commercially, produces a low-sulfur naphtha product in good yield which can be directly incorporated into the gasoline pool.